Charged drop generator with guard system

ABSTRACT

In apparatus for selectively charging of individual drops in a liquid stream or fluid filament divided into drops, a grounded guard electrode surrounds the stream between a charging electrode and a set of electrodes which create a deflection field. This isolates the fluid filament tip from the deflecting electrodes and avoids induced charges on it from the deflection field.

United States Patent Dale R. Beam Chillicothe, Ohio Nov. 24, 1969 May 4, 1971 The Mead Corporation Montgomery County, Ohio Continuation-impart of application Ser. No. 782,048, Dec. 9, 1968, now abandoned.

inventor Appl. No. Filed Patented Assignee CHARGED DROP GENERATOR WITH GUARD SYSTEM 1 Claim, 3 Drawing Figs.

US. Cl 346/75, 3 l 7/ 3 50 FieldofSearch 346/75; 317/3 [56] References Cited UNITED STATES PATENTS 2,676,868 4/1954 Jacob 346/75 3,298,030 1/1967 Lewisetal 346/75 3,373,437 3/1968 Sweetetal 346/75 3,416,153 12/1968 Hertzetal. 346/75 3,484,793 12/1969 Weigl 346/75 Primary Examiner-Joseph W. Hartary Attorney-Marechal, Biebel, French and Bug ABSTRACT: In apparatus for selectively charging of individual drops in a liquid stream or fluid filament divided into drops, a grounded guard electrode surrounds the stream between a charging electrode and a set of electrodes which create a deflection field. This isolates the fluid filament tip from the deflecting electrodes and avoids induced charges on it from the deflection field.

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INVENTOI? DALE R. BEAM BY WM M A TTOR/VE Y8 CHARGED DROP GENERATOR WITII GUARD SYSTEM RELATED APPLICATIONS BACKGROUND OF THE INVENTION This invention relates to the selective charging and control of drops in a stream or streams of liquid. It has been recognized that liquid discharged in a fine stream through a small orifice, e.g., l-3 mils diameter, will divide into drops in a regular pattern, and this action can be enhanced and regularized by appropriate stimulation at a desired frequency. As the drops separate, they are selectively electrostatically charged, and then may be passed through a deflection field wherein charged and uncharged drops will follow different tra ectories.

The switching potentials used on the drop charging electrodes are relatively low, in the order of 90 volts or less, while the potential between the deflecting electrodes may be in the order of several kilovolts. It has been observed that the deflection field may induce charges in the fluid filament and lead to false switching, particularly where the parts are constructed on a small scale and are separated only by thousandths of an inch, as in arrays of many closely spaced drop generators.

SUMMARY OFTHE INVENTION According to this invention a guard electrode is placed between the charging and deflection electrodes, thus isolating the fluid filament from the deflection field and avoiding spurious induced charges as aboveqme ntioned. Preferably the guard electrode is at ground potential, although some other bias potential may be selected if desired for a given set of circumstances. l

The object of the invention is therefore to minimize spurious induced charges in the fluid filament tip by use of such a guard electrode.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a diagram ofa typical drop generating unit;

FIG. 2 is a diagram of a modification; and

FIG. 3 is a schematic perspective view of the system shown in FIG. 2. 1

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, supply of liquid from a suitable source T and pumps means P is directed along pipe to an orifice l2, and issues as a fine stream or filament I4. Preferably a stimulating means such as vibrator I5 induces a regular perturbation in the filament, assuring that it separates into regularly sized, shaped, and spaced, individual drops 16. At the point where drop separation occurs, the filament is surrounded by a charging electrode in the form of ring 18. The location of the charging electrode relative to the orifice should be at the point at which the continuous stream from the orifice breaks into drops.

It has been observed that if the ring I8 is located too far from this point there is a loss of lcharge efficiency. In suitable embodiments constructed and successfully tested, an orifice of about 1.5 mils was used, with a flow rate of about 0.5 gallons per minute, and a stimulation frequency in the order of 50 kHLThe charging electrode ring had an inner diameter of 0.040 inch and was located 0.255inch below the orifice. These values are not limiting, but merely an example of one suitable embodiment. By applying a charge potential, the individual drops may be electrostatically charged.

Downstream along the path of the drops is a set of deflection electrodes 20, across which a suitable potential, for exampic I KV, is applied to create a deflection field. Charged drops passing through this field will be deflected, whereas uncharged drops will follow their original path. This switching capability can be used in either of two ways. The path of the uncharged drops can be considered the normal trajectory, and these drops can pass toa suitable receiving surface 22, while the charged drops are intercepted in a suitable catcher 23, or conversely the path of the deflected charged drops can be considercd the normal trajectory with the catcher used to intercept the uncharged drops. In either case, the drops following the designated normal trajectory can be deposited upon the receiving surface in a regular pattern. Suitable arrangements for control of the placement of the drops in space, or on receiving surfaces, are described in copending applications, all assigned to the assignee of this application, as follows: COOR- DINATE PLACEMENT OF INK DROPS, Ser. No. 768,766, filed Oct. I8, 1968, in the name of R. P. Taylor; IMAGE CONSTRUCTION SYSTEM USING MULTIPLE ARRAYS OF DROP GENERATORS, Ser. No. 768,790, filed Oct. I8, I968, in the names of R. P. Taylor, R. Van Brimer, and F. Culp; IMAGE CONSTRUCTION SYSTEM WITH SCANNING DROP GENERATORS, Ser. No. 768,800, filed Oct. I8, 1968, in the name of R. P. Taylor; and IMAGE CON- STRUCTION SYSTEM WITH CLOCKED INFORMATION INPUT, Ser. No. 768,763, filed Oct. 18, I968, in the names of drops to be deposited and the receiving member 22 may ,likewise be governed by appropriate controls such as explained in the above-mentioned copending applications.

It has been discovered that the field created by the deflecting electrodes particularly in constructions where the parts are all relatively small and closely spaced, may tend to induce charges in the tip of filanient 14, resulting in false switching. In order to avoid this situation, and thus to assure accurate charging control by the switching input, a guard electrode 25 in the form of a suitable electrically conducting ring, is placed around the path of the drops between the charge ring 18 and the deflecting electrodeslZO. Preferably the guard electrode is at ground potential, but in any event it is connected to a suitable bias source such that is shields the filament tip from spurit ous induced charges.

FIGS. 2 and 3 show the adaptation of this invention to an array of drop generators where close spacing makes it desirable to minimize the number and size of parts. As shown particularly in FIG. 3, an array of closely spaced drop generators is provided to project parallel streams of drops. It is understood that additional such arrays can be provided with the drop paths arranged to track adjacent lines on the web for more thorough coverage. Such constructions areshown and described in the aforementioned application Ser. No.

Liquid is pumped to the chamber 30 in a top bar 31, and passes to a plurality of orifices 32 formed in an orifice plate 33 fastened beneath the chamber. Each of the resulting liquid filaments 34 breaks into drops, and a stimulating means in the I form of a vibrator 35 causes the drops to form at a frequency which is common to all jfilaments, and results in drops of essentially equal size.

Beneath the orifice plate is a charging assembly in the form of a plate 37 having openings in which charging electrode rings 38 are carried, each ring being connected through a separate conductor 39 to an individual terminal in a terminal area 40 at one end of the plate 37. This assembly is conveniently formed by photoetching techniques such as used in manufacture of printed circuits, and the rings 38 extend through the substrate of the plate. Suitable electrical insulation (not shown) is provided between plates 33 and 37, such as by an insulating coating on the plate 37.

A spacer plate 42, of electrically insulating material, is fastened below the charging assembly, and provides at its ends a mounting place for terminal plugs 44 which have separate circuit connections for each conductor 39. These plugs engage receptacle units 45 which are connected through suitable cables 46 to the 'master control and switching unit 48 that in turn receives input signals representing intelligence to be reproduced on the moving web 50.

Below the spacer plate is the guard electrode plate 51, on top of the electrostatic deflection assembly comprising insulating support bars 52 mounting deflection electrodes 54 across which a substantial potential difference is applied, as indicated in FIG. 2. The guard electrode preferably is in the form of an insulating board having interconnected rings surrounding the apertures through the plate. The guard means could also be a conducting metallic plate having suitably located apertures. Uncharged drops follow a straight trajectory and deposit on web 50, which preferably is moved at constant velocity correlated to the drop generating frequency so that a continuous line of drops will cause an essentially continuous mark or trace along theweb. Charged drops will follow a curved trajectory due to the deflecting field, and these drops are removed from the system through a catcher assembly S5.

The catcher assembly includes a tube 56 connected to a vacuum pump 57, and a catcher blade 58 extending through a longitudinal slot in the tube. Screen or similar foraminous material is placed on opposite sides of blade 58, allowing drops of liquid which have collected on the blade to be drawn into the tube. 1 It should be understood that the entire assembly (FIG. 3) can be duplicated a number of times, with identical units spaced along the web and the drop generators of each assembly arranged to follow interleaved tracks on the web for thorough coverage of the desired image area. Due to the close spacing of the drop generators and the general compact construction, it is desirable to minimize the number of parts and their size. The shielding function can be obtained by grounding the individual charging electrodes 38 when they are not functioning to charge drops. a

Thus, the switching control transistor 60 is normally biased to a conducting state, providing a ground connection for its associated charging electrode 38. When an input signal is received over line 62 it is suitably amplified and applied to the base connection of the transistor 60, biasing it to cutoff. The potential of the charging electrode increases rapidly, and the next drop or drops separating from the liquid filament are charged. As soon as the input signal stops, the transistor 60 immediately conducts and grounds the charging electrode. The grounded guard plate 51 effectively shields the end of the liquid filament, and the separating drops, within the charging electrode from any effect of the deflection field.

While the forms of apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention defined in the following claims.

lclaim'.

1. ln apparatus for the precise placement of liquid drops on a receiving surface, said apparatus including an orifice plate with a plurality of equally sized orifices,

means for supplying liquid to all of said orifices at the same pressure,

means stimulating the resulting liquid filaments at a common frequency to produce parallel streams of equally spaced drops having the same size,

individual charging electrodes located adjacent each of the liquid filaments at the point where drops separate from the filaments, controls for the individual electrodes providing for selective charging of individual drops,

means providing a deflection field along the drop trajectories downstream of said charging electrodes, and

a catcher arranged to collect drops moved from their normal trajectory by action of the deflection field;

the improvement comprising interconnected grounded guard electrodes supported between each of said chargling electrodes and the deflection field to isolate each fluid filament from the deflection field and avoid inducing charges on the drops except by said charging electrodes. 

1. In apparatus for the precise placement of liquid drops on a receiving surface, said apparatus including an orifice plate with a plurality of equally sized orifices, means for supplying liquid to all of said orifices at the same pressure, means stimulating the resulting liquid filaments at a common frequency to produce parallel streams of equally spaced drops having the same size, individual charging electrodes located adjacent each of the liquid filaments at the point where drops separate from the filaments, controls for the individual electrodes providing for selective charging of individual drops, means providing a deflection field along the drop trajectories downstream of said charging electrodes, and a catcher arranged to collect drops moved from their normal trajectory by action of the deflection field; the improvement comprising interconnected grounded guard electrodes supported between each of said charging electrodes and the deflection field to isolate each fluid filament from the deflection field and avoid inducing charges on the drops except by said charging electrodes. 